1. Field of Invention
This invention relates to optical position sensors, and more particularly to fiber optic position sensors for measuring the linear displacement, or stroke, of a reciprocating device.
2. Description of the Prior Art
Fiber optic position sensors for measuring the present position of a reciprocating device to determine the particular displacement of the device within its stroke range at any given time, are known in the art. In general the position of the device is measured over the stroke through interruption of a light transmission path by an optical code plate having a selected code for providing a multiple bit signal value uniquely descriptive of the particular position within the stroke range. The resolution of the sensor is determined by the least significant bit (LSB) of the signal, which represents the minimum displacement to which the sensor may respond with a change in signal value.
The code plate comprises a metallized optical mask disposed through known photolithographic methods along one surface of a transparent substrate. The substrate is typically glass, and the mask is encoded on the glass with a series of transparent and opaque regions arrayed in a plurality of rows in a pattern determined by the code selected. Each row corresponds to one of the multiple bit channels providing the multiple bit optical signal. The bits extend from a most significant bit (MSB) to a least significant bit (LSB), and if a Gray code is selected for the mask, the bits provide sensed position information at a granularity which increases by a factor of two with each successive bit. The granularity of the LSB, which determines the maximum achievable resolution for the sensor, is determined by: Stroke Distance/2.sup.N ; where N equals the number of bit channels.
Typical of the prior art optical position sensors is that disclosed by U.S. Pat. No. 4,116,000 issued to Martin et al. and of common assignee herewith. As disclosed therein the light transmission path is established between a light transmitter and a light receiver disposed on either side of the code plate. The code plate is mechanically connected to the reciprocating device and moves in direct displacement therewith through the total stroke. The transmitter and receiver each include a plurality of light transmissive optic fibers, one associated with each of the bit channels of the code plate. The fibers of the transmitter and receiver associated with a common bit channel are in registration so as to permit light transmission therebetween. Since the light from each transmitting fiber diverges, the gap between the transmitter and receiver fibers establishes the spatial response distance of the sensor and also the degree of light detection error caused by light scattering and cross-talk within the substrate of the code plate. Therefore, in order to provide the maximum resolution the ends of the associated fibers of the transmitter and receiver must be placed as close as possible. As a result the code plates of the prior art sensors include very thin substrate material to ensure a minimum gap between the transmitter and receiver. This results in a fragile code plate structure which, while sufficient for measurement over a short stroke distance, is unsuitable for large stroke distances. In a high mechanical shock, or vibration environment, the longer stroke distances result in deformation of the code plate and, in the worst case, fracture of the plate resulting in catastrophic failure of the sensor.
For short stroke applications the substrate may be as thin as 250 microns with the metallized mask less than 1 micron. In order to provide a code plate having sufficient structural strength for use in the longer stroke applications the substrate thickness would have to be increased. This results in the increased occurrence of cross-talk, i.e. the light incident from the transmitting fiber of one bit channel being detected by the receiving fiber of the adjacent bit channel.